ARM: dts: armada-38x: use usb-nop-xceiv PHY for the xhci nodes on Armada 388 GP

Using the usb-nop-xceiv PHY for the xhci nodes allows a better
representation of the hardware but also a better handling of the
regulator. By linking the regulator to the PHY there is no more need to
use the regulator-always-on property, then it allows a better power
management.

The remaining usb node uses the ehci-orion driver which can't be used
with the usb-nop-xceiv PHY and must keeps the direct link to the
regulator with the regulator-always-on property.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>